The invention relates to fluorescent nanoparticles with particular suitability as in vivo diagnostic aid, especially as contrast agent for discriminating between different tissue types and claims the priority of European patent application 05 025 022.4, reference being made to the contents thereof.
In a large number of disorders, a diagnosis which is as early and informative as possible is crucially important for the choice and the harmonization and implementation of the necessary medical procedures. This applies in particular to a large number of tumor types, for whose determination and therapy (including possible sections) discrimination between healthy and carcinogenic tissue is essential. Accordingly, the recovery or even the survival of a patient depends crucially on whether and how well the treating and/or operating clinician can distinguish between different tissue types.
In the past, to improve diagnosis and the medical procedures, contrast agents with whose aid it is possible to visualize functions and structures in the body by imaging methods have been developed. These methods are used inter alia for targeted detection of cancer-associated cell alterations.
Thus, for example, Hsu et al. (2004) (“A far-red fluorescent contrast agent to image epidermal growth factor receptor expression”, Photochemistry and Photobiology, 79 (3): 272-279) have developed a molecular-specific contrast agent based on an organic fluorophore as marker for early carcinogenic transformation. In this case, the tumor-associated overexpression of the epidermal growth factor receptor (EGFR) is utilized for identifying altered tissue in the mouth via a red fluorescent anti-EGFR antibody conjugate (Alexa660).
A general disadvantage of organic fluorophores is that they are metabolized in the body, with the fluorochrome being degraded or inactivated. The metabolization thus counteracts the high labeling intensity which is necessary for diagnosis. As the residence time of the organic fluorophore in vivo increases, this problem intensifies and represents a considerable difficulty, especially in the labeling of cells in deeper tissue layers.
In addition, organic fluorophores which emit at longer wavelengths in particular have the disadvantage that their quantum yield is reduced by the chemical conjugation process. Moreover, organic fluorophores are very susceptible to photobleaching, which may even after brief irradiation lead to a substantial loss of fluorescence. A contrast agent based on these fluorophores thus has a fluorescence strength and stability with prolonged excitation time which are too low to be suitable for the detection/labeling of cells in deeper tissue layers (“deep tissue imaging”). Thus, it is evident from the study by Hsu et al. (2004) that the Alexa660 conjugates exhibit a maximum depth of penetration of 0.5 mm, so that detection of the fluorescence is no longer a reasonable possibility.
A further known possibility for the fluorescent labeling of cellular alterations consists of using so-called quantum dots (ODs), which are fluorescent nanoparticles a few nanometers in size whose core consists of semiconductor materials such as CdSe, CdTe, InP or the like.
However, when the known QDs are used in biological systems they show the so-called blinking phenomenon, i.e. the nanoparticles alternate between a fluorescent and a nonfluorescent state. This phenomenon makes the quantum dots useless in particular for in vivo application. In addition, the “blinking” may also indicate a disintegration of the nanoparticle core, by means of which toxic cadmium may be released into the body. This is particularly disadvantageous because the quantum dots accumulate in the body, for example in the liver or the spleen.